1. Field of the Invention
The present invention relates to cooling water used in a continuous annealing process of cold-rolled steel strips and the like.
2. Description of the Prior Arts
As a method for preventing the formation of oxide film on the surface of steel strips in the course of this continuous annealing process, it has been known to add what is generally called, organic acids, to cooling water used in water quenching.
The organic acids used for the above purpose in the prior arts include: unbranched chain fatty acids, such as formic acid, acetic acid, propionic acid, oxalic acid, and succinic acid; oxyacids such as citric acid, lactic acid, gluconic acid and tartaric acid; and nitrilotriacetic acid, ethylenediaminetetraacetic acid.2 sodium, as disclosed in Japanese Patent Publication No. Sho 57-47738. Among the above acids, nitrilotriacetic acid and ethylenediaminetetraacetic acid belong to the family of aminopolycarboxylic acids, not to the group of amino acids, and have a completely different chemical nature from amino acids.
Also Japanese Laid-Open Patent Specification No. Sho 57-85923 discloses cooling agents for metals composed of water-soluble organic acid and water-soluble organic amine, and as preferred, water-soluble organic acid, specifies water-soluble dicarboxylic acids having three or more carbon atoms, such as saturated dicarboxylic acids including malonic acid, succinic acid, glutaric acid, adipic acid, and pimelic acid; non-saturated dicarboxylic acids, such as maleic acid, itaconic acid; and hydroxycarboxylic acids, such as malic acid and tartaric acid.
Further, Japanese Laid-Open Patent Application No. Sho 58-55533 discloses a quenching process with use of an aqueous solution containing organic acids, such as malonic acid, formic acid, citric acid, acetic acid, lactic acid, succinic acid and tartaric acid.
All of the various organic acids disclosed in the prior art publications have been found to be not satisfactory; some are not satisfactorilly effective to prevent the formation of oxide film, and others make the removal of oxide film difficult, depending on the temperature of the solution or on the temperature of the cold-rolled steel strip after cooling.